Optical phosphate glass in which the length of the light path is temperature independent

ABSTRACT

A phosphate optical glass, whose length of the light path is substantially independent of temperature, comprising, in % by weight, 
     
         ______________________________________                                    
 
    
     P 2  O 5       27 - 55                                              
K 2  O            6 - 26                                               
BaO                  10 - 50                                              
K 2  O + BaO      larger than 35                                       
Al 2  O 3      2 - 15                                               
TiO 2  + Nb 2  O 5  + Ta 2  O 5                            
                     4 - 20                                               
TiO 2             0 - 14                                               
Nb 2  O 5      0 - 20                                               
Ta 2  O 5      0 - 7                                                
B 2  O 3       0 - 5                                                
Li 2  O           0 - 2                                                
Na 2  O           0 - 5                                                
MgO                  0 - 6                                                
CaO                  0 - 8                                                
SrO                  0 - 10                                               
ZnO                  0 - 10                                               
PbO                  0 - 13, and                                          
As 2  O 3      0 - 2 .                                              
______________________________________

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical phosphate glass in which thetemperature dependence of the length of the light path is nearly zeroand which has a small Abbe number.

2. Description of the Prior Art

It is well known that when a temperature gradient occurs in glass, thelength of the light path changes, and the wave front is distorted in theinterior of the glass. This reduces the ability of the glass to form afocused image. For example, when a glass is used in a laser system, atemperature gradient is developed inside the glass by localized heatinginduced by the laser beam, and consequently, the beam is distorted. Whenglass is exposed to extreme temperature changes, for example, opticalinstruments set in a spacecraft, the length of the light path changesdue to the temperature gradient generated in the glass, and a distortionoccurs in the wave front, which in turn results in a reduction in theability of the glass to form a focused image. Therefore, it would bevery advantageous to construct an optical system using a glass in whichthe length of the light path is quite independent of temperature.

Changes in the length of the light path based on temperature changes ina glass sheet whose optical surfaces are parallel planes can becalculated using the following equation.

    .increment. W.sub.G = [α(n-1) + dn/dT]·l·.increment. T

or if

    .increment. W.sub.G = G · l · .increment. T

then,

    G = α(n-1) + dn/dT

wherein:

α IS THE COEFFICIENT OF THERMAL EXPANSION OF THE GLASS (IN ° C⁻¹);

n is the refractive index of the glass;

L IS THE THICKNESS OF THE GLASS SHEET, AND

.increment.T represents the temperature change (in ° C).

When .increment. w_(G) changes due to the occurrence of a temperaturegradient in various parts of the glass sheet then a distortion occurs inthe wave front. In the above equation, the term

    G = α(n-1) + dn/dT

is the part of the equation at which the change of the length of thelight path is determined by the composition of the glass sheet. Hence,by reducing G to 0, the change in the length of the light path based onthe temperature gradient in the glass can be removed. For this purpose,dn/dT should be a negative value since α(n-1) normally has a positivevalue.

Separately, there is a change in the length of the light path which iscaused by thermal stress in the glass. This change is shown by thefollowing equation.

    .increment. W.sub.s = ∂n/∂σ·dσ/dT·l.multidot..increment.T

wherein σ represents the thermal stress, n is the refractive index ofthe glass, l is the thickness of the glass and .increment. T is thetemperature change.

Since .increment.W_(s) normally has a positive value, in order to reducethe change of the total length of the light path to zero, i.e.,

    .increment. W = .increment.W.sub.G + .increment.W.sub.s = 0

.increment. W_(G) must be <0, and hence, G < 0.

Glasses having a small Abbe number and where the length of the lightpath is independent of temperature must be rendered achromatic bycombining glasses having a large Abbe number.

Japanese Patent Application (OPI) No. 37109/76 discloses a glass havingan Abbe number νd in the range of 30 to 45 and where the length of thelight path is nearly independent of temperature. This glass, however, isa fluoroborosilicate glass, and poses many manufacturing problemsbecause the glass contains fluorine.

Japanese Patent Publication 1091/73 and Japanese Patent Application(OPI) No. 37108/76 also disclose a glass which is free from atemperature dependence. The glass disclosed in Japanese PatentPublication 1091/73 has a refractive index of 1.5 or less and an Abbenumber of 50 to 65 and contains fluorine which involves manymanufacturing problems as in Japanese Patent Application (OPI) No.37109/76. Further, the glass disclosed in Japanese Patent Application(OPI) No. 37108/76 has a refractive index of 1.59 to 1.65 and an Abbenumber of 55 to 46, which are narrower than those of this invention.

SUMMARY OF THE INVENTION

According to the present invention, an optical glass is obtained whichhas an Abbe number in the range of 51 to 33, which has a refractiveindex (nd) of 1.57 to 1.68, and whose length of the light path isindependent of temperature. The present invention is based on thediscovery that in a phosphate glass, K₂ O and BaO are components whichcause the dn/dT to have a high negative value, and that in phosphateglasses containing these components in great amounts, even when TiO₂,Nb₂ O₃ and Ta₂ O₅ are added in considerable amounts, the dn/dT stillremains a high negative value, and the g value does not exceed +1.5 ×10⁻⁶ /° C.

Accordingly, the present invention provides a phosphate optical glass,whose length of the light path is substantially temperature independent,which comprises in % by weight,

    ______________________________________                                        P.sub.2 O.sub.5      27 - 55                                                  K.sub.2 O            6 - 26                                                   BaO                  10 - 50                                                  K.sub.2 O + BaO      larger than 35                                           Al.sub.2 O.sub.3     2 - 15                                                   TiO.sub.2 + Nb.sub.2 O.sub.5 + Ta.sub.2 O.sub.5                                                    4 - 20                                                   TiO.sub.2            0 - 14                                                   Nb.sub.2 O.sub.5     0 - 20                                                   Ta.sub.2 O.sub.5     0 - 7                                                    B.sub.2 O.sub.3      0 - 5                                                    Li.sub.2 O           0 - 2                                                    Na.sub.2 O           0 - 5                                                    MgO                  0 - 6                                                    CaO                  0 - 8                                                    SrO                  0 - 10                                                   ZnO                  0 - 10                                                   PbO                  0 - 13, and                                              As.sub.2 O.sub.3     0 - 2 .                                                  ______________________________________                                    

DETAILED DESCRIPTION OF THE INVENTION

P₂ O₅ is a glass-forming oxide essentially providing a large α, and anegative dn/dT value to the glass. In order to obtain a stable glass, P₂O₅ should be present in an amount of 27 to 55%.

K₂ O and BaO are components essential for causing the dn/dT to be highlynegative and the total amount of these components should be larger than35%. If the amount of K₂ O is less than 6% by weight, a devitrificationof the glass tends to occur. If the amount of K₂ O is larger than 25%,the α value of the glass tends to be too large. BaO is less effectivethan K₂ O in causing the dn/dT value to be negative but BaO provides achemical resistance which is higher and it has less of an effect inincreasing the α value. Hence, BaO is also an effective component and asuitable amount of BaO is 10 to 50%.

Al₂ O₃ stabilizes the glass, increases the chemical resistance of theglass and reduces the α value, but increases the dn/dT value. Hence, asuitable amount of Al₂ O₃ is 2 to 15%.

The total amount of TiO₂ + Nb₂ O₅ + Ta₂ O₅ should be 4 to 20%. TiO₂ isquite effective in reducing the Abbe number of the glass, but if TiO₂ isused in an amount greater than 14%, devitrification of the glass tendsto occur. A violet color may sometimes form due to the presence of Ti³⁺,but this color can be removed by adding 0 to 2% of As₂ O₃. Nb₂ O₅ has agreater tendency to stabilize the glass than does TiO₂, and can be addedin an amount of up to 20%. Coloration by Nb₂ O₅ does not pose a problem.Ta₂ O₅ can be added in an amount of up to 7% without affecting thestability of the glass.

B₂ O₃, in an amount of up to 5%, is effective for stabilizing the glass.When P₂ O₅ is replaced by B₂ O₃, the coloration due to Ti³⁺ can bemarkedly reduced, but B₂ O₃ somewhat reduces the chemical resistance ofthe glass. Li₂ O, when used as a replacement for K₂ O, reduces the αvalue of the glass, but increases the dn/dT value of the glass. Hence, asuitable amount of Li₂ O is up to 2%. Likewise, Na₂ O can be introducedin an amount of up to 5%.

MgO, CaO, SrO, and ZnO are effective as components for reducing the αvalue of the glass when the amount thereof is up to 6%, 8%, 10% and 10%,respectively. If these amounts exceed the limits specified above,devitrification of the glass tends to occur, and the dn/dT valueincreases.

PbO can be introduced without increasing the dn/dT value very much. PbOreduces the νd of the glass, but increases the nd markedly. Accordingly,a suitable amount of PbO is up to 13%.

Some representative examples of the glass of this invention and theirphysical properties are shown in the following tables. However, thepresent invention should not be construed as being limited to theseexamples. All percents are by weight.

                                      Table 1                                     __________________________________________________________________________                  Example No.                                                     Component     1   2   3   4   5   6                                           __________________________________________________________________________    P.sub.2 O.sub.5                                                                             52.6                                                                              43.3                                                                              30.8                                                                              33.2                                                                              41.2                                                                              32.7                                        K.sub.2 O     15.9                                                                              25.0                                                                              7.7 8.3 12.2                                                                              8.1                                         BaO           20.7                                                                              11.7                                                                              50.0                                                                              44.8                                                                              34.8                                                                              44.1                                        Al.sub.2 O.sub.3                                                                             3.4                                                                               7.8                                                                              2.8 9.0  6.6                                                                              5.9                                         TiO.sub.2      5.4                                                                              12.2                                                                              8.7 4.7  5.2                                                                              9.2                                         Nb.sub.2 O.sub.5                                                                            --  --  --  --  --  --                                          Ta.sub.2 O.sub.3                                                                            --  --  --  --  --  --                                          B.sub.2 O.sub.3                                                                             --  --  --  --  --  --                                          Li.sub.2 O    --  --  --  --  --  --                                          Na.sub.2 O    --  --  --  --  --  --                                          MgO           --  --  --  --  --  --                                          CaO           --  --  --  --  --  --                                          SrO           --  --  --  --  --  --                                          ZnO           --  --  --  --  --  --                                          PbO           --  --  --  --  --  --                                          As.sub.2 O.sub.3                                                                             2.0                                                                              --  --  --  --  --                                          nd            1.5735                                                                            1.6000                                                                            1.6612                                                                            1.6106                                                                            1.5978                                                                            1.6501                                      νd         43.8                                                                              34.7                                                                              38.0                                                                              50.4                                                                              46.2                                                                              40.8                                        α 20-40° C(×10.sup.-7 /° C)                                       141 140 122 133 128 116                                         dn/dT20-40° C(×10.sup.-6 /° C)                                          -10.2                                                                             -9.0                                                                              -9.4                                                                              -8.5                                                                              -8.7                                                                              -6.8                                        G 20 -40° C(×10.sup.-6 /° C)                                            -2.1                                                                              -0.6                                                                              -1.3                                                                              -0.4                                                                              -1.0                                                                              +0.7                                        Da            3   2   4   2   3   3                                                         Example No.                                                     Component     7   8   9   10  11  12                                          __________________________________________________________________________    P.sub.2 O.sub.5                                                                             30.8                                                                              34.8                                                                              29.5                                                                              31.9                                                                              36.7                                                                              35.9                                        K.sub.2 O      7.7                                                                              14.4                                                                               7.3                                                                              7.9 15.2                                                                              14.9                                        BaO           41.6                                                                              28.2                                                                              39.9                                                                              43.1                                                                              29.8                                                                              29.1                                        Al.sub.2 O.sub.3                                                                             5.5                                                                               6.3                                                                               5.3                                                                              5.7  6.6                                                                               6.4                                        TiO.sub.2     --  --  --  5.4  8.3                                                                               8.1                                        Nb.sub.2 O.sub.5                                                                            14.4                                                                              16.3                                                                              18.0                                                                              6.0  3.4                                                                              --                                          Ta.sub.2 O.sub.3                                                                            --  --  --  --  --   5.6                                        B.sub.2 O.sub.3                                                                             --  --  --  --  --  --                                          Li.sub.2 O    --  --  --  --  --  --                                          Na.sub.2 O    --  --  --  --  --  --                                          MgO           --  --  --  --  --  --                                          CaO           --  --  --  --  --  --                                          SrO           --  --  --  --  --  --                                          ZnO           --  --  --  --  --  --                                          PbO           --  --  --  --  --  --                                          As.sub.2 O.sub.3                                                                            --  --  --  --  --  --                                          nd            1.6471                                                                            1.6164                                                                            1.6647                                                                            1.6495                                                                            1.6243                                                                            1.6202                                      νd         45.0                                                                              44.5                                                                              41.7                                                                              42.9                                                                              38.5                                                                              39.4                                        α 20-40° C(×10.sup.-7 /° C)                                       119 128 114 118 128 132                                         dn/dT20-40° C(×10.sup.- 6 /° C)                                         -7.5                                                                              -8.5                                                                              -6.7                                                                              -7.1                                                                              -8.1                                                                              -8.1                                        G 20-40° C(×10.sup.-6 /° C)                                             +0.2                                                                              -0.6                                                                              +0.9                                                                              +0.6                                                                              -0.1                                                                              +0.1                                        Da            3   3   3   3   2   3                                                         Example No.                                                     Component     13  14  15  16  17  18                                          __________________________________________________________________________    P.sub.2 O.sub.5                                                                             39.5                                                                              36.0                                                                              29.9                                                                              32.0                                                                              32.6                                                                              34.3                                        K.sub.2 O     16.4                                                                              16.6                                                                              8.3 14.7                                                                              12.0                                                                               9.5                                        BaO           25.6                                                                              23.9                                                                              44.8                                                                              38.4                                                                              39.2                                                                              36.0                                        Al.sub.2 O.sub.3                                                                             3.5                                                                               7.2                                                                              6.0  3.2                                                                               3.3                                                                               3.4                                        TiO.sub.2     11.1                                                                              11.3                                                                              9.4 10.0                                                                              10.1                                                                              10.7                                        Nb.sub.2 O.sub.5                                                                            --  --  --  --  --  --                                          Ta.sub.2 O.sub.3                                                                            --  --  --  --  --  --                                          B.sub.2 O.sub.3                                                                              3.9                                                                               4.0                                                                              1.6  1.7                                                                               1.8                                                                               1.9                                        Li.sub.2 O    --  --  --  --   1.0                                                                              --                                          Na.sub.2 O    --  --  --  --  --   4.2                                        MgO           --  --  --  --  --  --                                          CaO           --  --  --  --  --  --                                          SrO           --  --  --  --  --  --                                          ZnO           --  --  --  --  --  --                                          PbO           --  --  --  --  --  --                                          As.sub.2 O.sub.3                                                                            --   1.0                                                                              --  --  --  --                                          nd            1.6257                                                                            1.6201                                                                            1.6515                                                                            1.6404                                                                            1.6543                                                                            1.6527                                      νd         34.5                                                                              38.0                                                                              41.8                                                                              38.8                                                                              37.5                                                                              37.0                                        α 20-40° C(×10.sup.-7 /° C)                                       127 117 118 139 126 138                                         dn/dT20-40° C(×10.sup.-6 /° C)                                          -8.0                                                                              -6.7                                                                              -6.4                                                                              -10.4                                                                             -8.9                                                                              -8.2                                        G 20-40° C(×10.sup.-6 /° C)                                             -0.1                                                                              +0.5                                                                              +1.3                                                                              -1.5                                                                              -0.7                                                                              +0.8                                        Da            4   4   3   4   3   4                                                         Example No.                                                     Component     19  20  21  22  23  24                                          __________________________________________________________________________    P.sub.2 O.sub.5                                                                             34.4                                                                              35.5                                                                              36.0                                                                              29.9                                                                              27.8                                                                              32.9                                        K.sub.2 O     15.9                                                                              16.3                                                                              16.6                                                                              11.3                                                                              10.6                                                                              12.1                                        BaO           24.8                                                                              25.5                                                                              26.0                                                                              36.9                                                                              34.4                                                                              34.4                                        Al.sub.2 O.sub.3                                                                             3.4                                                                               3.5                                                                               3.6                                                                               6.1                                                                               5.7                                                                               3.3                                        TiO.sub.2     10.8                                                                              11.1                                                                              11.3                                                                               9.6                                                                               9.0                                                                              10.3                                        Nb.sub.2 O.sub.5                                                                            --  --  --  --  --  --                                          Ta.sub.2 O.sub.3                                                                            --  --  --  --  --  --                                          B.sub.2 O.sub.3                                                                              1.9                                                                               1.9                                                                               2.0                                                                              --  --   1.8                                        Li.sub.2 O    --  --  --  --  --  --                                          Na.sub.2 O    --  --  --  --  --  --                                          MgO           --  --   4.5                                                                              --  --  --                                          CaO           --   6.2                                                                              --  --  --  --                                          SrO           --  --  --   6.2                                                                              --  --                                          ZnO            8.8                                                                              --  --  --  --   5.2                                        PbO           --  --  --  --  12.5                                                                              --                                          As.sub.2 O.sub.3                                                                            --  --  --  --  --  --                                          nd            1.6413                                                                            1.6386                                                                            1.6286                                                                            1.6384                                                                            1.6708                                                                            1.6530                                      νd         36.1                                                                              37.2                                                                              38.2                                                                              42.9                                                                              37.3                                                                              37.7                                        α 20-40° C(×10.sup.-7 /° C)                                       120 123 119 125 137 117                                         dn/dT20-40° C(×10.sup.-6 /° C)                                          -6.6                                                                              -7.0                                                                              -6.0                                                                              -7.7                                                                              -7.9                                                                              -6.9                                        G 20-40° C(×10.sup.-6 /° C)                                             +1.1                                                                              +0.9                                                                              +1.5                                                                              +0.3                                                                              +1.3                                                                              +0.7                                        Da            4   3   3   3   3   3                                           __________________________________________________________________________

In the tables the values for (1) α 20°-40° C, (2)dn/dT 20°-40° C, and(3) G 20°-40° C are those at a temperature of 20° to 40° C, and the Darepresents the weight loss (wt%) of the glass and was evaluated in thefollowing manner:

Pulverized glass having a particle size of 420 to 590 microns wascharged into a flask made of a quartz glass containing a 0.1N--HNO₃aqueous solution and the flask was then immersed in a boiling water bathfor 1 hr. The weight loss (wt%) was then classified into the followinggrades.

    __________________________________________________________________________              Grade                                                                         1   2    3    4    5    6                                           __________________________________________________________________________    Weight Loss (wt%)                                                                       ≦0.19                                                                      0.20-0.34                                                                          0.35-0.64                                                                          0.65-1.19                                                                          1.20-2.19                                                                          ≧2.20                                __________________________________________________________________________

The glass of this invention has superior chemical resistance toconventional glasses having optical constants similar to those of theglass of this invention and in which the temperature dependence of thelength of the light path is nearly zero, as shown in Table 2 below wherethe chemical resistance of glasses of this invention and those of theprior art were compared.

                  Table 2                                                         ______________________________________                                                  Example No.                                                                   2      37109-3  8        37109-4                                    ______________________________________                                        nd          1.6201   1.6153   1.6164 1.6038                                   νd       38.0     39.9     44.5   43.0                                     dn/dT 20-40° C                                                                     -6.7     -4.2     -8.5   -5.3                                     (×10.sup.-6 /° C)                                                G 20-40° C                                                                         +0.5     +2.0     -0.6   +1.0                                     (×10.sup.-6 /° C)                                                Da          4        5        3      5                                        ______________________________________                                    

In Table 2 above, 37109-3 and 37109-4 mean Examples 3 and 4 of JapanesePatent Application (OPI) No. 37109/76, respectively.

Further, the glass of this invention has an additional characteristicthat the stability to crystallization is high, which makes it possibleto prepare a large block of glass, e.g., having a size of 300 × 200 × 50mm, or more.

To prepare the glasses of this invention and in the above examples, anorthophosphoric acid and various phosphates are used as a material forP₂ O₅, and for the other components, starting materials generallysuitable for optical purposes are used. The components are mixed andmelting of the glass is performed in a platinum melter at a temperatureof about 1200° to about 1300° C. Desirably, the melting is carried outin an oxidizing atmosphere to inhibit reducing reactions.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A phosphate optical glass having an Abbe numberin the range of 51 to 33, a refractive index (nd) of 1.57 to 1.68, andwhose length of the light path is substantially independent oftemperature, consisting essentially of in % by weight,

    ______________________________________                                        P.sub.2 O.sub.5      27 - 55                                                  K.sub.2 O            6 - 26                                                   BaO                  10 - 50                                                  K.sub.2 O + BaO      larger than 35                                           Al.sub.2 O.sub.3     2 - 15                                                   TiO.sub.2 + Nb.sub.2 O.sub.5 + Ta.sub.2 O.sub.5                                                    4 - 20                                                   TiO.sub.2            0 - 14                                                   Nb.sub.2 O.sub.5     0 - 20                                                   Ta.sub.2 O.sub.5     0 - 7                                                    B.sub.2 O.sub.3      0 - 5                                                    Li.sub.2 O           0 - 2                                                    Na.sub.2 O           0 - 5                                                    MgO                  0 - 6                                                    CaO                  0 - 8                                                    SrO                  0 - 10                                                   ZnO                  0 - 10                                                   PbO                  0 - 13, and                                              As.sub.2 O.sub.3     0 - 2 .                                                  ______________________________________                                    